SIZE-DEPENDENT CATALYTIC ACTIVITY OF SUPPORTED METAL-CLUSTERS

BECAUSE catalysis by metals is a surface phenomenon, many technologica l catalysts contain small (typically nanometre-sized) supported metal particles with a large fraction of the atoms exposed(1). Many reaction s, such as hydrocarbon hydrogenations, are structure-insensitive, proc eeding at approximately the same rate on metal particles of various si zes provided that they are larger than about 1 nm and show bulk-like m etallic behaviour(1). But it is not known whether the catalytic proper ties of metal particles become size-dependent as the particles become so small that they are no longer metallic in character. Here we invest igate the catalytic behaviour of precisely defined clusters of just fo ur and six iridium atoms on solid supports. We find that the Ir-4 and Ir-6 clusters differ in catalytic activity both from each other and fr om metallic Ir particles. This raises the possibility of tailoring the catalytic behaviour of metal clusters by controlling the cluster size .